Specific techniques for the encapsulation of electronic components within a protective synthetic resinous material are well known in the art and are widely practiced. See, for instance, the article entitled "Encapsulation of Semiconductor Devices" appearing in Plastics Design Forum Issue, April, 1981, at Pages 49 to 54.
Heretofore, thermosetting resinous materials commonly have been employed to bring about the desired encapsulation through a form of injection molding commonly termed transfer molding. For instance, epoxy resins (e.g. novolac-hardened epoxy systems) commonly have been employed for this purpose. Also, thermosetting resins such as unsaturated polyesters, bis-imido polymers, etc., have been proposed for use as encapsulation materials. See, for instance U.S. Pat. Nos. 4,327,369; 4,374,080; and 4,390,596. Such thermosetting materials often require refrigeration prior to use, tend to require relatively long cycle times during molding, and after molding must be cured for extended periods of time at elevated temperatures in an oven before the desired cure of the encapsulant is complete. Since the viscosity of the uncured thermosetting resin increases with curing as the resin is heated, relatively prompt use of the uncured material must be made once heating is commenced. With such materials the recycle of scrap is impossible. Additionally, such materials have naturally occurring resin binders commonly have a tendency to flash and to adhere to the surfaces of the mold cavity causing possible mold damage thus requiring the substantial remedial attention of skilled personnel during the course of a molding run. This may preclude fully automating the encapsulation process.
While not utilized on a commercial scale, certain thermoplastic resins have heretofore been proposed for use in the encapsulation of electronic components. See, for instance, U.S. Pat. No. 4,327,369 at Col. 6, lines 18 to 23, where passing reference is found to polyvinyl chloride, polyolefins, such as low-density polyethylene, high density polyethylene, polypropylene, and polystyrene. See also, U.S. Pat. No. 4,370,292 where a polyphenylene sulfide composition which includes a phenoxy resin is proposed for encapsulation.
It is a common practice of the prior art to include within the encapsulating resin a particulate filler material such as silica or alumina which serves among other things to increase the thermal conductivity and to decrease the volumetric coefficient of thermal expansion of the molded composition. Such particulate fillers, however, greatly modify (i.e. increase) the viscosity of the composition during molding especially when present in high concentrations. If the viscosity becomes too great the molding composition becomes difficult to cause to flow and to satisfactorily fill the mold. If voids are present in the molded article the encapsulation will be considered a failure. If the viscous composition is caused to flow through increased pressure, this may damage the delicate electronic component undergoing encapsulation. This damage is termed "wire sweep" in U.S. Pat. No. 4,374,080. Such sweep or deflection may severly stress or break the electrical circuit as bonds are torn or cause deleterious shorting. Also if one attempts to achieve the requisite viscosity for the molding composition through the reduction of the molecular weight of a thermoplastic encapsulant, then the resulting molded article commonly will possess inadequate mechanical properties (e.g. brittleness). If the polymeric material possesses contamination inherent in many polymerization processes (e.g. water-extractable halogens or water-extractable ionic materials), such contamination may attack the encapsulated electronic component and/or adversely influence its operation. Also, if the polymeric material has a propensity substantially to evolve gaseous by-products for any reason (e.g. because of a condensation polymerization reaction) during the molding operation or during subsequent use of the encapsulated electronic component, this can lead to excessive voids and the failure of the encapsulant composition. In addition to the above shortcomings many thermoplastics are incapable of prolonged reliable service at the elevated temperatures commonly encountered by electronic components and/or readily burn when subjected to flame thereby diminishing their protective properties.
It is an object of the present invention to provide an improved molding composition which is particularly suited for use in the impervious encapsulation of an electronic component.
It is an object of the present invention to provide an improved thermoplastic molding composition which is particularly suited for the encapsulation of a delicate electronic component such as a quad or a dual-in-line integrated circuit.
It is an object of the present invention to provide an improved thermoplastic molding composition containing a substantial quantity of a particulate inorganic material which is capable of encapsulating a delicate electronic component without damage to yield a final encapsulated product which exhibits highly satisfactory mechanical, thermal, chemical, and electrical properties which render it capable of satisfactory service for an extended period of time even if adverse environmental conditions are encountered.
It is an object of the present invention to provide an improved method for encapsulating an electronic component which can be carried out without the need for a time consuming polymer curing step as commonly practiced in the prior art.
It is another object of the present invention to provide an improved encapsulated electronic component.
It is a further object of the present invention to provide an improved encapsulated electronic component wherein the filled encapsulating resin is substantially impervious to water and ultraviolet light, is substantially void-free, exhibits satisfactory mechanical properties (e.g. mechanical strength), exhibits a satisfactory volumetric coefficient of thermal expansion, exhibits a satisfactory thermal conductivity, contains less than 50 parts per million of water-extractable alkali metal, contains less than 100 parts per million of water-extractable halogen, exhibits a V-0 burning rating when subjected to the UL94 test, and is capable of extended service.
These and other objects as well as the scope, nature, and utilization of the claimed invention will be apparent to those skilled in the art from the following detailed description and appended claims.